diff --git a/client/space_lua/stdlib.ts b/client/space_lua/stdlib.ts
index a112103e..b4fe8b30 100644
--- a/client/space_lua/stdlib.ts
+++ b/client/space_lua/stdlib.ts
@@ -1,4 +1,5 @@
 import {
+  getMetatable,
   type ILuaFunction,
   isILuaFunction,
   isLuaTable,
@@ -17,6 +18,7 @@ import {
   luaToString,
   luaTypeOf,
   type LuaValue,
+  singleResult,
 } from "./runtime.ts";
 import { stringApi } from "./stdlib/string.ts";
 import { tableApi } from "./stdlib/table.ts";
@@ -32,6 +34,7 @@ import { luaLoad } from "./stdlib/load.ts";
 import { cryptoApi } from "./stdlib/crypto.ts";
 import { netApi } from "./stdlib/net.ts";
 import { isTaggedFloat, makeLuaFloat } from "./numeric.ts";
+import { isPromise } from "./rp.ts";
 
 const printFunction = new LuaBuiltinFunction(async (_sf, ...args) => {
   console.log(
@@ -135,9 +138,35 @@ const typeFunction = new LuaBuiltinFunction(
   },
 );
 
-const tostringFunction = new LuaBuiltinFunction((_sf, value: any) => {
-  return luaToString(value);
-});
+// tostring() checks `__tostring` metamethod first (with live SF), then
+// falls back to the default `luaToString` representation.
+const tostringFunction = new LuaBuiltinFunction(
+  (sf, value: any): string | Promise<string> => {
+    const mt = getMetatable(value, sf);
+    if (mt) {
+      const mm = mt.rawGet("__tostring");
+      if (mm !== undefined && mm !== null) {
+        const ctx = sf.astCtx ?? {};
+        const r = luaCall(mm, [value], ctx as any, sf);
+        const unwrap = (v: any): string => {
+          const s = singleResult(v);
+          if (typeof s !== "string") {
+            throw new LuaRuntimeError(
+              "'__tostring' must return a string",
+              sf,
+            );
+          }
+          return s;
+        };
+        if (isPromise(r)) {
+          return (r as Promise<any>).then(unwrap);
+        }
+        return unwrap(r);
+      }
+    }
+    return luaToString(value);
+  },
+);
 
 const tonumberFunction = new LuaBuiltinFunction(
   (sf, value: LuaValue, base?: number) => {
diff --git a/client/space_lua/stdlib/global_test.lua b/client/space_lua/stdlib/global_test.lua
index e2ef5318..9d1cdc1f 100644
--- a/client/space_lua/stdlib/global_test.lua
+++ b/client/space_lua/stdlib/global_test.lua
@@ -25,3 +25,30 @@ assertEqual(some(" \n"), nil)
 assertEqual(some({}), nil)
 assertEqual(some({"baz"})[1], "baz") -- compare an element to ensure passthrough
 assertEqual(some({foo="bar"})["foo"], "bar")
+
+-- tostring: primitives
+assertEqual(tostring(nil), "nil")
+assertEqual(tostring(true), "true")
+assertEqual(tostring(false), "false")
+assertEqual(tostring(42), "42")
+assertEqual(tostring(3.14), "3.14")
+
+-- tostring: `__tostring` metamethod is called with the live SF
+local mt = { __tostring = function(t) return "custom:" .. t.name end }
+local obj = setmetatable({ name = "lua" }, mt)
+assertEqual(tostring(obj), "custom:lua")
+
+-- tostring: `__tostring` returning non-string must error
+local bad_mt = { __tostring = function(_) return 99 end }
+local bad_obj = setmetatable({}, bad_mt)
+local ok, err = pcall(tostring, bad_obj)
+assertEqual(ok, false)
+
+-- tostring: `__tostring` on a nested call (SF propagation via `luaCall`)
+local inner_mt = {
+    __tostring = function(t)
+        return "inner:" .. tostring(t.val)
+    end
+}
+local inner = setmetatable({ val = 7 }, inner_mt)
+assertEqual(tostring(inner), "inner:7")